This invention relates to a catalyst system for the preparation of olefin copolymers. The catalyst system is especially useful for the preparation of ethylene copolymers which have very high molecular weight and very low density. The catalyst system is characterized by the use of an organometallic complex having a ketimide ligand.
Ketimide complexes of group 4 metals have been reported in the literaturexe2x80x94see, for example, the review of titanium chemistry which as prepared in part by one of us in 1982 (Ref: M. Bottrill, P. D. Gavens, J. W. Kelland and J. MeMeeking in xe2x80x9cComprehensive Organometallic Chemistryxe2x80x9d, Ed. G. Wilkinson, F. G. A. Stone, and E. W. Abel, Pergamon Press, 1982, Section 22.3, page 392). However, the use of ketimide ligand/group 4 metal complex as an ethylene polymerizafion catalyst was heretofore unknown.
Preferred ketimide catalysts of this invention also contain one and only one cyclopentadienyl-type ligand.
The prior art includes many examples of olefin polymerization catalysts having a single cyclopentadienyl ligandxe2x80x94most notably the so called Bercaw ligand (*Cp-Me2Sixe2x80x94NtBu) which was disclosed as a Scandium complex by Bercaw et al In the fall of 1988 and subsequently claimed as a titanium complex in U.S. Pat. No. 5,064,802 (Stevens and Neithamer, to Dow Chemical) and U.S. Pat. No. 5,055,438 (Canich, to Exxon). The use of a titanium complex of the Bercaw ligand provides an olefin polymerization catalyst which has excellent commoner responsexe2x80x94i.e. the catalyst Is excellent for the preparation of ethylene/xcex1-olefin copolymers. However, the bridged structure of the Bercaw ligand is expensive to synthesize. Accordingly, an olefin polymerization catalyst which doesn""t require a xe2x80x9cbridgexe2x80x9d to provide commoner response would represent a useful addition to the art.
The present invention also provides a catalyst system for olefin polymerization comprising;
(a) a catalyst which is an organometallic complex of a group 4 metal; and
(b) an activator, characterized in that such organometallic complex contains a ketimide ligand.
Preferred forms of the catalyst contain a single ketimide ligand and a single cyclopentadienyl-type ligand.
The invention further provides a process for the copolymerization of ethylene with at least one other olefin monomer using the above described catalyst system.
The term xe2x80x9cgroup 4xe2x80x9d metal refers to conventional IUPAG nomenclature. The preferred group 4 metals are Ti, Hf and Zr with Ti being most preferred.
As used herein, the term xe2x80x9cketimide ligandxe2x80x9d refers to a ligand which:
(a) is bonded to the group 4 metal via a metal-nitrogen atom bond,
(b) has a single substituent on the nitrogen atom, (where this single substituent Is a carbon atom which is doubly bonded to the N atom); and.
(c) has two substituents (Sub 1 and Sub 2, described below) which are bonded to the carbon atom.
Conditions a, b, and c are illustrated below: 
The substituents xe2x80x9cSub 1 and Sub 2xe2x80x9d may be the same or different. Exemplary substituents include hydrocarbyls having from 1 to 20 carbon atoms; silyl groups, amido groups and phosphido groups. For reasons of cost and convenience it is preferred that these substituents both be hydrocarbyls, especially simple alkyls and most preferably tertiary butyl.
In the preferred catalyst systems, the catalyst is defined by the formula:
L1L2MX2xe2x80x83xe2x80x83formula 1
L2:
L2 is a cyclic ligand which forms a delocalized pi-bond with the group 4 metal. L2 is preferably a cyclopentadienyltype ligand.
As used herein, the term cyclopentadienyl-type is meant to convey its conventional meaning and to include indenyl and fluorenyl ligands. The simplest (unsubstituted) cyclopentadione indeno and fluorene structures are illustrated below. 
Ligands in which one of the carbon atoms in the ring is replaced with a phosphorous atom (i.e. a phosphole) may also be employed
It will be readily appreciated by those skilled in the art that the hydrogen atoms shown in the above formula may be replaced with substituents to provide the xe2x80x9csubstitutedxe2x80x9d analogues, Thus, the preferred catalysts contain a cyclopentadienyl structure which may be an unsubstituted cyclopentadienyl, substituted cyclopentadienyl, unsubstituted indenyl, substituted indenyl, unsubstituted fluorenyl or substituted fluorenyl. A description of permissible substituents on these cyclopentadienyl-type structures is provided in U.S. Pat. No. 5,324,800 (Welbom).
An illustrative list of such substituents for cyclopentadienyl groups included C1-C20 hydrocarbyl radicals; substituted C1-C20 hydrocarbyl radicals wherein one or more hydrogen atoms is replaced by a halogen radical, an amido radical, a phosphido radical, an alkoxy radical or a radical containing a Lewis acidic or basic functionality; substituted C1-C20 hydrocarbyl radicals wherein the substituent contains an atom selected from the group 14 of the Periodic Table of Elements (where group 14 refers to IUPAC nomenclature); and halogen radicals, amido radicals, phosphido radicals, alkoxy radicals, alkyborido radicals, or a radical containing Lewis acidic or basic functionality; or a ring in which two adjacent R-groups are joined forming C1-C20 ring to give a saturated or unsaturated polyclinic ligand.
Ligand X: xe2x80x9cNon-Interfering Anionic Ligandxe2x80x9d
Referring for formula 1, the preferred catalyst systems according to this invention contain two simple anionic ligands denoted by the letter X.
Any simple anionic ligand which may be bonded to an analogous metallocene catalyst component ((i.e. where the analogous metallocene catalyst component is defined by the formula Cp2M(X)2, where Cp is a cyclopentadienyl-type ligand; M Is a group 4 metal; and X is a non-interfering ligand Is previously defined herein) may also be used with the catalyst components or this invention.
xe2x80x9cNon-interferingxe2x80x9d means that this ligand doesn""t interfere with (deactivate) the catalyst.
An Illustrative list includes hydrogen, hydrocarbyl having up to 10 carbon atoms, halogen, amido and phosphido (with each X preferably being chlorine, for simplicity).
Polymerization Details
The polymerization process of this invention is conducted in the presence of a catalyst and an xe2x80x9cactivator or cocatalystxe2x80x9d. The terms xe2x80x9cactivatorxe2x80x9d or xe2x80x9ccocatalystxe2x80x9d may be used interchangeably and refer to a catalyst component which combines with the organometallic complex to form a catalyst system that is active for olefin polymerization.
Preferred cocatalysts are the well know alumoxane (also known as aluminoxane) and ionic activators.
The term xe2x80x9calumoxanexe2x80x9d refers to a well known article of commerce which is typically represented by the following formula:
R2xe2x80x2AlO(Rxe2x80x2AlO)mAlR2xe2x80x2
were each Rxe2x80x2 is independently selected from alkyl, cycloalkyl, aryl or alkyl substituted aryl and has from 1-20 carbon atoms and where m is from 0 to about 50 (especially from 10 to 40). The preferred alumoxane is methylalumoxane or xe2x80x9cMAOxe2x80x9d (where each of the Rxe2x80x2 is methyl).
Alumoxanes are typically used in substantial molar excess compared to the amount of metal in the catalyst. Aluminum:transition metal molar ratios of from 10:1 to 10,000:1 are preferred, especially from 50:1 to 500:1.
Another type of activator is the xe2x80x9cionic activatorxe2x80x9d or xe2x80x9csubstantially non-coordinating anionxe2x80x9d. As used herein, the term substantially non-coordinating anions (xe2x80x9cSNCAxe2x80x9d) well known cocatalyst or activator systems which are described, for example, in U.S. Pat. No. 5,153,157 (Hlatky and Turner), and the carbonium, sulfonium and oxonium analogues of such activators which are disclosed by Ewen in U.S. Pat. No. 5.387.568. In general, these SNCA form an anion which only weakly coordinates to a cationic form of the catalyst.
While not wanting to be bound by theory, it is generally believed that SNCA-type activators ionize the catalyst by abstraction or protonation of one of the xe2x80x9cXxe2x80x9d ligands (non-interfering ligands) so as to ionize the group 4 metal center into a cation (but not to covalently bond with the group 4 metal) and to provide sufficient distance between the ionized group 4 metal and the activator to permit a pulverizable olefin to enter the resulting active site. It will appreciated by those skilled in the art that it is preferable that the xe2x80x9cnon-interferingxe2x80x9d(xe2x80x9cXxe2x80x9d) ligands be simple alkyls when using a SNCA as the activator. This may be achieved by the alkylation of a halide form of the catalyst.
Examples of compounds capable of ionizing the group 4 metal complex include the following compounds:
triethylammonium tetra(phenyl)boron,
tripropylammonium tetra(phenyl)boron,
tri(n-butyl)ammonium tetra(phenyl)boron,
trimethylammonium tetra(p-tolyl)boron,
trimethylammonium tetra(o-tolyl) boron,
tributylammonium tetra(pentafluorophenyl)boron,
tripropylammonium tetra(o,p-dimethylphenyl)boron,
tributylammonium tetra(m,m-dimethylphenyl)boron,
tributylammonium tetra(p-trifluoromethylphenyl)boron,
tribulylammonium tetra(pentafluorophenyl)boron,
tri(n-butyl)ammonium tetra(o-tolyl)boron,
N,N-dimethylanilinium tetra(phenyl)boron,
N,N-diethylanilinium tetra(phenyl)boron,
N,N-diethylanilinium tetra(phenyl)n-butylboron,
N,N-2,4,6-pentamethylanilinium tetra(phenyl)boron,
di-(isopropyl)ammonium tetra(pentafluorophenyl)boron,
dicyclohexylammonium tetra(phenyl)boron,
triphenylphosphonium tetra(phenyl)boron,
tri(dimethylphenyl)phosphonium tetra(phenyl)boron,
tri(dimethylphenyl)phosphonium tetra(phenyl)boron,
tropillium tetrakispentafluorophenyl borate,
triphenylmethylium tetrakispentafluorophenyl borate,
benzene (diazonium) tetrakispentafluorophenyl borate,
tropillium phenyltris-pentafluorophenyl borate,
triphenylmethylium phenyl-trispentafluorophenyl borate,
benzene (diazonium) phenyltrispentafluorophenyl borate,
tropillium tetrakis(2,3,5,6-tetrafluorophenyl) borate,
triphenylmethylium tetrakis(2,3,5,6-tetrafluorophenyl) borate,
benzene (diazonium) tetrakis(3,4,5-trifluorophenyl) borate,
tropilliurm tetrakis(3,4,5-trifluorophenyl) borate,
benzene (diazonium) tetrakis(3,4,5-trifluorophenyl) borate,
tropillium tetrakis(1,2,2-trifluoroethenyl) borate,
triphenylmethylium tetrakis(1,2,2-trifluoroethenyl) borate,
benzene (diazonium) tetrakis(1,2,2-trifluoroethenyl) borate,
tropillium tetrakis (2,3,4,5-tetrafluorophenyl) borate,
triphenylmethylium tetrakis(2,3,4,5-tetrafluorophenyl)
borate, and
benzene (diazonium) tetrakis(2,3,4,5-tetrafiuorophenyl) borate.
Readily commercially available activators which are capable of ionizing the group 4 metal complexes include;
N,N-dimethylaniliumtetrakispentafluorophenyl borate
(xe2x80x9c[Me2NHPh][B(C6F5)4]xe2x80x9d); and
triphenylmethylium tetrakispentafluorophenyl borate (xe2x80x9c[Ph3C][B(C6F5)4]xe2x80x9d); and
trispentafluorophenyl boron.
These SNCA activators are typically used in approximately equimolar amounts (based on the group 4 metal in the catalyst) but lower levels may also be successful and higher levels also generally work (though sub-optimally with respect to the cost-effective use of the expensive activator).
In addition to the catalyst and cocatalyst, the use of a xe2x80x9cpoison scavengersxe2x80x9d may also be desirable. As many be inferred from the name xe2x80x9cpoison scavengerxe2x80x9d. these additives may be used in small amounts to scavenge impurities in the polymerization environment. Aluminum alkyls, for example trisobutyl aluminum, are suitable poison scavengers. (Note: some caution must be exercised when using poison scavengers as they may also react with, and. deactivate, the catalyst.)
Polymerizations according to this invention may be undertaken in any of the well know olefin polymerization processes including those known as xe2x80x9cgas phasexe2x80x9d, xe2x80x9cslurryxe2x80x9d, xe2x80x9chigh pressurexe2x80x9d and xe2x80x9csolutionxe2x80x9d.
The use of a supported catalyst is preferred for gas phase and slurry processes whereas a non-supported catalyst is preferred for the solution process.
When utilizing a supported catalyst, it may be preferable to initially support the cocatalyst, then the catalyst (as will be illustrated in the Examples).
The polymerization process according to this invention uses ethylene and may include other monomers which are copolymerizable therewith (such as other alpha olefins, preferably butene, hexene or octene, and under certain conditions, dienes such as hexadiene isomers, vinyl aromatic monomers such as styrene or cyclic olefin monomers such as norbornene).
The present invention may also be used to prepare elastomeric co- and terpolymers of ethylene, propylene and optionally one or more diene monomers. Generally. such elastomeric polymers will contain about 50 to abut 75 weight % ethylene, preferably about 50 to 60 weight % ethylene and correspondingly from 50 to 25 % of propylene. A portion of the monomers, typically the propylene monomer, may be replaced by a conjugated diolefin. The diolefin may be present in amounts up to 10 weight % of the polymer although typically is present in amounts from about 3 to 5 weight %. The resulting polymer may have a composition comprising from 40 to 75 weight % of ethylene, from 50 to 15 weight % of propylene and up to 10 weight % of a diene monomer to provide 100 weight % of the polymer. Preferred but not limiting examples of the dienes are dicyclopentadliene, 1,4-hexadiene, 5-methylene-2-norbornene, 5-ethylidene-2-norbornene and 5-vinyl-2-norbornene. Particularly preferred dienes are 5-ethylidene-2-norbornene and 1,4-hexadiene.
The polyethylene polymers which may be prepared in accordance with the present invention typically comprise not less than 60, preferably not less than 70 weight % of ethylene and the balance one or more Cr4-10 alpha olefins, preferably selected from the group consisting of 1-butene, 1-hexene and 1-octene. The polyethylene prepared in accordance with the present invention may be linear low density polyethylene having density from about 0.910 to 0.935 g/cc. The present invention might also be useful to prepare polyethylene having a density below 0.910 g/ccxe2x80x94the so called very low and ultra low density polyethylenes.
The most preferred polymerization process of this invention encompasses the use of the novel catalysts (together with a cocatalyst) in a medium pressure solution process. As used herein, the term xe2x80x9cmedium pressure solution processxe2x80x9d refers to a polymerization carried out in a solvent for the polymer at an operating temperature from 100 to 320xc2x0 C. (especially from 120 to 220xc2x0 C.) and a total pressure of from 3 to 35 mega Pascals. Hydrogen may be used in this process to control (reduce) molecular welgnt. Optimal catalyst and cocatalyst concentrations are affected by such variables as temperature and monomer concentration but may be quickly optimized by non-inventive tests.
Further details concerning the medium pressure polymerization process are well known lo those skilled in the art and widely described in the open and patent literature.
The catalyst of this invention may also be used in a slurry polymerization process or a gas phase polymerization process.
The typical slurry polymerization process uses total reactor pressures of up to about 50 bars and reactor temperature of up to about 200xc2x0 C. The process employs a liquid medium (e.g. an aromatic such as toluene or an alkane such as hexane, propane or isobutane) In which the polymerization take place. This results in a suspension of solid polymer particles in the medium. Loop reactors are widely used in slurry processes. Detailed descriptions of slurry polymerization processes are widely reported in the open and patent literature.
In general, a fluidized bed gas phase polymerization reactor employs a xe2x80x9cbedxe2x80x9d of polymer and catalyst which is fluidized by a flow of monomer which is at least partially gaseous. Heat is generated by the enthalpy of polymerization of the monomer flowing through the bed. Unreacted monomer exits the fluidized bed and is contacted with a cooling system to remove this heat. The cooled monomer is then re-circulated through the polymerization zone together with xe2x80x9cmake-upxe2x80x9d monomer to replace that which was polymerized on the previous pass As will be appreciated by those skilled in the art, the xe2x80x9cfluidizedxe2x80x9d nature of the polymerization bed helps to evenly distribute/mix the heat of reaction and thereby minimize the formation of localized temperature gradients (or xe2x80x9chot spotsxe2x80x9d). Nonetheless, it is essential that the heat of reation be properly removed so as to avoid softening or melting of the polymer (and the resultant-and highly undesirablexe2x80x94xe2x80x9creactor chunksxe2x80x9d). The obvious way to maintain good mixing and cooling is to have a very high monomer flow through the bed. However, extremely high monomer flow causes undesirable polymer entrainment.
An alternative (and preferable) approach to high monomer flow is the use of an inert condensable fluid which will boil in the fluidized bed (when exposed to the enthalpy of polymerization), then exit the fluidized bed as a gas, then come into contact with a cooling element which condenses the inert fluid. The condensed, cooled fluid is then returned to the polymerization zone and the boiling/condensing cycle is repeated.
The above-described use of a condensable fluid additive in a gas phase polymerization is often referred to by those skilled in the art as xe2x80x9ccondensed mode operationxe2x80x9d and is described in additional detail in U.S. Pat. No. 4,543,399 and U.S. Pat No. 5,352,749. As noted in the ""399 reference, it is permissible to use alkanes such as butane, pentanes or hexanes as the condensable fluid and the amount of such condensed fluid preferably does not exceed about 20 weight per cent of the gas phase.
Other reaction conditions for the polymerization of ethylene which are reported in the ""399 reference are: Preferred Polymerization Temperatures: about 75xc2x0 C. to about 115xc2x0 C. (with the lower temperatures being preferred for lower melting copolymersxe2x80x94especially those having densities of less than 0.915 g/ccxe2x80x94and the higher temperatures being preferred for higher density copolymers and homopolymers); and Pressure: up to abut 1000 psi (with a preferred range of from about 100 to 350 psi for olefin polymerization).
The ""399 reference teaches that the fluidized bed process is well adapted for the preparation of polyethylene but further notes that other monomers may also be employed. The present invention is similar with respect to choice of monomers.
Catalysts which are used in gas phase and slurry polymerizations are preferably supported. An exemplary list of support materials include metal oxides (such as silica, alumina, silica-alumina, titania and zirconia); metal chlorides (such as magnesium chloride); talc, polymers (including polyolefins); partially prepolymerized mixtures of a group 4 metal complex, activator and polymer; spray dried mixtures of the group 4 metal complex, activator and fine xe2x80x9cinertxe2x80x9d particles (as disclosed, for example, in European Patent Office Application 668,295 (to Union Carbide).
The preferred support material is silica. In a particularly preferred embodiment, the silica has been treated with an alumoxane (especially methylalumoxane or xe2x80x9cMAOxe2x80x9d) prior to the deposition of the group 4 metal complex. The procedure for preparing xe2x80x9csupported MAOxe2x80x9d which is described in U.S. Pat. No. 5,534,474 (to Wltco) may provide a low cost catalyst support. It will be recognized by those skilled in the art that silica may be characterized by such parameters as particle size, pore volume and residual silanol concentration. The pore size and silanol concentration may be altered by heat treatment or calcining. The residual silanol groups provide a potential reaction site between me alumoxane and the silica (and, indeed, some off gassing is observed when alumoxane is reacted with silica having residual silanol groups). This reaction may help to xe2x80x9canchorxe2x80x9d the alumoxane to the silica (which, in turn, may help to reduce reactor fouling).
The preferred particle size, preferred pore volume and preferred residual silanol concentration may be influenced by reactor conditions. Typical silicas are dry powders having a particle size of from i to 200 microns (with an average particle size of from 30 to 100 being especially suitable); pore size from 50 to 500 Angstroms; and pore volumes of from 0.5 to 5.0 cubic centimeters per gram. As a general guideline, the use of commercially available silicas, such as those sold by W. R. Grace under the trademarks Davison 948 or Davison 955, are suitable.